In these 4 billion years that have
passed since Earth's inception, there have been many significant incidents that have
threatened the lives of living beings. In fact, some scientists estimate that there have been as
many as 20 extinction events throughout Earth's history. However, let's focus on the "big five,"
five times when circumstances became exceedingly challenging for life on our planet.
Ordovician-Silurian Extinction Event
Around 445 million years ago, during the
end of the Ordovician period, life in the oceans was thriving with all sorts of weird and
wonderful creatures like conodonts, trilobites, and early fish. North America was mostly
underwater back then, so if you desired a swim, you'd have some interesting company. Life
had been evolving and diversifying, setting the stage for many future species to come.
But then, things took a dark turn. Two major events rocked the oceans, resulting in a massive
extinction that led to the demise of a whopping 85% of all species over about a million years.
No significant meteor or volcano can be blamed this time, though. One popular theory suggests
that the supercontinent Gondwana decided to take a trip down south, causing glaciers to form
rapidly and sea levels to drop dramatically. And just like that, many of the amazing creatures
that once roamed the ancient seas were gone.
Some scientists believe it was due to the oceans
losing a significant amount of oxygen, making them highly toxic. Others theorize that it might have
been caused by a gamma-ray burst, essentially a massive explosion of gamma radiation from a star
going supernova. These bursts are incredibly rare but immensely powerful, potentially damaging the
ozone layer and bombarding us with harmful rays. It could be a combination of both these theories
that led to the mass extinction event we see in the fossil record. Regardless, only about
15% of species survived, and it took millions of years for life to rebound.
Late Devonian Extinction Event
Let's jump forward about 70 million years to the
Late Devonian period. At this time, animals were starting to inhabit land entirely alongside basic
plants that were quickly spreading. The oceans were also bustling with life, featuring armored
fish like the intimidating Dunkleosteus, a massive predator weighing over 600 kilograms. But the
Dunkleosteus and many other species were about to face serious trouble. During the Late Devonian,
a series of events shook up Earth's biodiversity. The timeline of these events is up for debate,
with estimates ranging from 500,000 years to over 20 million years (quite a range indeed). The exact
details of what occurred are also fuzzy since the evidence is somewhat scarce and theories differ.
there's this intriguing theory floating around that back in the Late Devonian period, plants
might have disrupted the whole atmosphere by absorbing too much carbon dioxide. Normally,
carbon dioxide in the air is associated with warming the planet, but in this case,
it might have actually caused a cooling effect. Some individuals also suggest that a large
meteorite, like perhaps the one that formed the Siljan Ring in Sweden, could have contributed
to all this upheaval. But who really knows, right? What we do know for certain is that this
was a challenging time for life on Earth. Coral reefs were decimated, armored fish vanished, and
so did those sophisticated new ammonites. Around 70% of all species succumbed during this period.
Moreover, due to low oxygen levels in the oceans, organic matter had difficulty decomposing,
eventually leading to the creation of oil reserves in Canada and the US. The Late Devonian
was essentially a disaster zone for life, and although land creatures fared relatively
better, everyone experienced a turbulent time during the subsequent extinction event.
Permian-Triassic Extinction Event
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way back around 250 million years ago, there was this cataclysmic event called the
Permian-Triassic Extinction. People also call it "the Great Dying" for a reason - it wiped
out a multitude of species, including the tough trilobites that had been around forever.
It lasted for a staggering 50,000 years, making it the deadliest extinction event ever.
Now, what caused all this chaos? Well, first off, there were these super intense volcanic eruptions
in Siberia that lasted a long time. They emitted copious amounts of sulfur dioxide and carbon
dioxide, disrupting the planet's temperature and acidifying the oceans. These weren't
your average volcanoes - they were massive, covering an area larger than Mexico with lava!
And here's the kicker, they even ignited huge underground oil and coal deposits, releasing even
more greenhouse gases into the air. Additionally, there were new microorganisms in the mix producing
methane or releasing it from the seafloor.
Pangaea, the supercontinent, was forming, which
some speculate disrupted the oceans and weather patterns, leading to a myriad of marine life going
extinct. Then there's British astrophysicist John Gribbin's theory. He suggests that when our solar
system traversed a spiral arm of the Milky Way galaxy, a plethora of interstellar dust obscured
the Sun, inducing an Ice Age. Most species on land perished, and in the oceans, it was even worse
- with over 95% of marine life going extinct. Even insects weren't spared, underscoring
the severity of the situation. But hey, some resilient creatures like the archosaurs,
ancestors of crocodiles and dinosaurs, managed to survive. The Triassic period witnessed
the ascendancy of dinosaurs, who began to dominate the scene alongside other formidable
creatures like Eryops and Kaprosuchus.
Triassic-Jurassic Extinction Event
So, back in the Triassic-Jurassic era, things took a turn for the wild in the animal
kingdom. Approximately 70% of species bid farewell and disappeared in less than 10,000 years. The
primary suspect? A volcanic upheaval known as the Central Atlantic Magmatic Province (CAMP),
which spewed copious amounts of carbon dioxide into the air and disrupted the oceans' pH levels
– a lethal combination for survival. Oh, and naturally, there's speculation about a meteoric
impact on Earth, with some pointing fingers at the Manicouagan Reservoir in Quebec. It's hypothesized
that this meteor might have fragmented before impact, causing multiple strikes worldwide, akin
to a cosmic bowling ball strike on a global scale. Despite the chaos, nature rebounded swiftly
from the extinction event. Dinosaurs asserted, "Hey, we're still here!" and took over the
world, demonstrating life's resilience when afforded the opportunity to thrive.
Cretaceous-Paleogene Extinction Event
so, the dinosaurs had a significant run, didn't
they? I mean, they thrived for approximately 140 million years, which is remarkably long. But, as
they say, all good things must come to an end. Around 66 million years ago, there occurred
the Cretaceous-Paleogene Extinction event, or KT for short, and that essentially
marked the end for the dinosaurs.
Unlike some other extinction events where
uncertainties abound, scientists are quite confident about what transpired during KT.
Luis Alvarez, a notable figure who even won a Nobel Prize in Physics, proposed the idea that a
massive asteroid collided with Earth at the end of the Cretaceous Period. He and his son Walter
observed that the rock layer from that time contained a significant amount of iridium, which
is indicative of asteroid impacts. Subsequently, they discovered the massive Chicxulub Crater in
the Yucatan Peninsula, which aligned perfectly with the timing of the dinosaurs' disappearance.
This asteroid was formidable, to say the least. It measured approximately 15 km in diameter,
taller than Mount Everest if placed at sea level. When it struck Earth at a speed of 20 km
per second, it unleashed an explosion billions of times more powerful than the Hiroshima bomb. This
impact disrupted the climate, ejected ash into the atmosphere, and triggered numerous volcanic
eruptions. And just like that, it was curtains for the dinosaurs. Consequently, the oceans became
acidic, greatly impacting most marine life. Furthermore, the Sun's radiation was obstructed,
leaving plants devoid of the much-needed sunlight. With no vegetation to sustain themselves,
herbivores began to perish, followed by the carnivores who relied on them. Before long,
75% of all living creatures had vanished.
But amidst this chaos, guess who rose to the
occasion? Mammals! These creatures had been on the sidelines for eons, but with the dinosaurs
out of the picture, they finally had their moment. Within a few million years after the KT
Extinction event, mammals underwent a remarkable transformation, evolving into incredible creatures
like whales, dolphins, and primates. And thus, that paved the way for the ascent of
humans, the dominant species on the planet.
